d. Scour Protection . The forces causing loss of foundation soil from 

 beneath a rubble-mound structure are accentuated at the structure toe. Wave 

 pressure differentials and groundwater flow may produce a quick condition at 

 the toe, then currents may carry the suspended soil away. A shallow scour 

 hole may remove support for the cover layers, allowing them to slump down the 

 face, while a deep hole may destabilize the slope of the structure, over- 

 steepening it until bearing failure in the foundation soil allows the whole 

 face to slip. Toe protection in the form of an apron must prevent such damage 

 while remaining in place under wave and current forces and conforming to an 

 uneven bottom that may be changing as erosion occurs. 



Toe scour is a complex process. The toe apron width and stone size 

 required to prevent it are related to the wave and current intensity; the 

 bottom material; and the slope, roughness, and shape of the structure face. 

 No definitive method for designing toe protection is known, but some general 

 guidelines for planning toe protection are given below. The guidelines will 

 provide only approximate quantities which may require doubling to be 

 conservative, in some cases. A detailed study of scour in the natural bottom 

 and near existing structures should be conducted at a planned site, and model 

 studies should be considered before determining a final design. 



(1) Minimum Design . Hales (1980) surveyed scour protection practices 

 in the United States and found that the minimum toe apron was an extension of 

 the bedding layer and any accompanying filter blanket measuring 0.6 to 1.0 

 meter thick and 1.5 meters wide. In the northwest United States, including 

 Alaska, aprons are commonly 1.0 to 1.5 meters thick and 3.0 to 7.5 meters 

 wide. Materials used, for example, were bedding of quarry-run stone up to 0.3 

 meter in dimension or of gabions 0.3 meter thick; core stone was used if 

 larger than the bedding and required for stability against wave and current 

 forces at the toe. 



(2) Design for Maximum Scour Force . The maximum scour force occurs 

 where wave downrush on the structure face extends to the toe. Based on Eckert 

 (1983), the minimum toe apron will be inadequate protection against wave scour 

 if the following two conditions hold. The first is the occurrence of water 

 depth at the toe that is less than twice the height of the maximum expected 

 unbroken wave that can exist in that water depth. The maximum unbroken wave 

 is discussed in Chapter 5 and is calculated using the maximum significant wave 

 height Hg^ from Figure 3-21, and methods described in Section I of this 

 chapter. Available wave data can be used to determine which calculated wave 

 heights can actually be expected for different water levels at the site. 



The second condition that precludes the use of a minimum toe apron is a 

 structure wave reflection coefficient x that equals or exceeds 0.25, which 

 is generally true for slopes steeper than about 1 on 3. If the reflection 

 coefficient is lower than the limit, much of the wave force will be dissipated 

 on the structure face and the minimum apron width may be adequate. If the toe 

 apron is exposed above the water, especially if waves break directly on it, 

 the minimum quarrystone weight will be inadequate, whatever the slope. 



(3) Tested Designs . Movable bed model tests of toe scour protection 

 for a quarrystone-armored jetty with a slope of 1 on 1.25 were performed by 

 Lee (1970; 1972). The tests demonstrated that a layer two stones thick of 



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